This invention relates to self-charging electronic devices which, when not in use, rest in a charging cradle. The charging cradle provides both a location to store the device and the circuitry and electrical contacts to recharge the self-charging electronic device when it is not in use. Specifically, this invention relates to an apparatus and method for preventing the batteries in the electronic device from being continually charged while resting in the charging cradle.
Self-charging electronic devices such as cordless telephones, drills, screw drivers and other devices are well known. These self-charging devices include a rechargeable batteries or other rechargeable power supplies, and therefore do not have to be connected to a power supply by permanent cords or wiring. These devices, therefore, can be operated at locations remote from a power supply, and are particularly useful outdoors or in hard to reach places. When not in use, self-charging devices typically rest in a charging cradle or other device which provides both a place to store the device and the electrical circuitry and electrical contacts to recharge the batteries in the self-charging device.
The electronic devices can be powered by rechargeable batteries, such as sealed lead acid or nickel-cadmium ("nicad") batteries. Nicad batteries have become increasingly popular due to their ability to maintain a charge for a significant period of time, their ability to supply sufficient current to power remote electronic devices, and their small size. The use of nicad batteries has made rechargeable electronic devices smaller and easier for consumers to handle.
Nicad batteries, however, suffer from several disadvantages. Specifically, the charge and discharge cycles of nicad batteries exhibit a "memory" effect which may prevent the battery from fully charging after the initial use. For example, nicad batteries may not recharge to full capacity if they are not completely discharged before a recharging current is applied. In fact, the first discharge after a long period of charging may provide as little as 50% of the batteries nominal capacity.
To prevent the memory problem, it is necessary to allow the battery to discharge to a low charge level before applying a charging current, and then apply a charging current to restore the battery to fill strength. When stored in a charging cradle, however, the battery is continually recharged, regardless of the charge level of the battery.
It is therefore an object of the invention to extend the available use time of the rechargeable battery in a self-charging electronic device.
It is still another object of the invention to limit the memory effects on a rechargeable battery in a self-charging device.
It is yet another object of the invention to selectively apply charging current to a self-charging electronic device when the electronic device is resting in a charging cradle.
It is still another object of the invention to prevent the unwanted charging of a rechargeable battery in a self-charging electronic device.
It is a still further object of the invention to interrupt the charging current between a first set of charging contacts located in a charging cradle and a second set of charging contacts located in a self-charging electronic device.
It is yet a further object of the invention to provide a removable isolation plate which can be positioned between a first set of charging contacts located in a charging cradle and a second set of charging contacts located in a self-charging electronic device.